1. Field
The exemplary embodiments generally relate to water processing and treatment, and more particularly, to the determination of electrochemically active ions in an aqueous solution.
2. Brief Description of Related Developments
In recent years there has been increasing demand for continuous real-time or near real-time monitoring of solution composition. Of particular interest are voltammetric detectors which measure the current response at given applied potential. Voltammetric detectors have applications which cover many fields and include for example environmental monitoring, process control, and biomedical monitoring. In particular, voltammetric detectors have found applications in heavy metal monitoring, clinical chemistry and as detectors for use in high-performance liquid chromatography HPLC.
Many other techniques are also currently available for the detection of contaminants. The development and improvement of these techniques has become a major focal point of analytical science because of the growing need to detect very small amounts of contaminants which adversely affect the environment. For example, mercury is regarded as a very toxic heavy metal, and its presence in soil and waterways represents a considerable health hazard. Government agencies throughout the world are increasing restrictions on the release of mercury to the environment. In some countries, a legislated limit of 2 parts-per-billion in drinking water has been enforced. Other potentially hazardous metals like lead and cadmium appear to be receiving the same scrutiny. The United States Environmental Protection Agency is lowering the allowable level of arsenic from 50 parts per billion down to 10 parts per billion or perhaps as low as 2 parts per billion in drinking water and in discharge permits.
The most commonly used methods for detecting various trace contaminants are atomic absorption (AA), inductively coupled plasma atomic emission (ICP-AE), and mass spectroscopy (MS). Each of these methods is suitable for trace analysis of nonmetals, metalloids, and metals, for example mercury in a laboratory setting. However, they often require well-controlled experimental conditions, expensive instrumentation, and frequent maintenance and calibration. Moreover, these methods usually require lengthy sample preparation, especially when other interfering elements or impurities are present in the sample under investigation. For these reasons, the methods mentioned above are not particularly well-suited for rapid analysis in the field or on-site in a treatment plant. Other methods which are sometimes suitable for contaminant detection and analysis in the field include X-Ray Fluorescence (XRF), colorimetry, and ion-selective electrodes (ISE). Special mention is made of XRF, which is used in the field because of its suitability for simultaneously detecting many contaminants without substantial sample preparation. However, the detection limits for this method (about 30-100 ppm) is not low enough for accurately determining very low levels of metals like mercury (2 ppb). Moreover, XRF is very dependent on the nature of the environmental sample. For example, if one is running a mercury analysis on both a soil sample and a plastic sample, a separate calibration curve must be prepared for each. Colorimetric techniques can be complicated and time-consuming. Also, such techniques are often very specific, e.g. selective to only one type of mercury complex, unlike the exemplary embodiment described herein, which are sensitive to all electro active species of an element.
One significant disadvantage of most commonly used methods in the detection of trace contaminants is the difficulty of performing analyses of highly complex samples, such as ocean water. In complex solutions there can be a wide variety of elements with concentration levels much higher than the contaminants of concern which often interfere with the accurate detection and quantification of trace elements. The concentration difference between the contaminants of concern and the other impurities in the water precludes the successful application of many analytical tools and techniques. The analysis of complex waters, such as ocean waters, by common methods requires the extraction of the contaminant of concern from the sample before an accurate analysis is made, e.g. in ocean water, one would have to separate the salts from the ions to be analyzed. One distinct advantage to the proposed voltammetric based system is that the effect of interference is minimized with comparatively little to no sample preparation required. Presently, many common methods frequently require extensive sample pretreatment to determine low impurity levels of highly complex samples. Consequently most analytical determinations are made off-line in a conventional laboratory setting.
Voltammetric detectors offer considerable advantages in terms of sensitivity and selectivity over other techniques mentioned above. Stripping Voltammetry (SV) techniques cathodic and also anodic, as well as potentiometric analysis (PA) have long been used in trace analysis. In stripping voltammetry, the electroactive species in the sample are first pre-concentrated on the working electrode surface using a controlled potential or potentials. Once the ions are electrochemically collected on the face of the working electrode, the potential is varied to strip the material from the electrode surface. The current used and produced while stripping the material from the electrode surface is proportional to the concentration of the electro-active species in the sample. Electrodes for SV comprise a working electrode, reference electrode (usually Ag/AgCl), and an auxiliary (counter) electrode, usually platinum or graphite. The system and process of the exemplary embodiments is designed to analyze samples with complex matrices, and the system is designed to eliminate any possible interferences.
Thus, prior art systems are mostly for laboratory use, labor intensive and require considerable supervision by skilled personnel in order to determine low levels of contaminant concentrations. Furthermore, conventional techniques are impaired by interference caused by high concentrations of other species present with the impurities. If interference is expected in conventional techniques, it is often necessary to alter the electrolyte by the addition of suitable substances to avoid interference.
U.S. Pat. No. 4,804,443, entitled, “METHOD AND APPARATUS FOR THE DETERMINATION OF ELECTROCHEMICALLY ACTIVE COMPONENTS IN A PROCESS STREAM”, to Newman, et al., is effective in analysis of samples with high concentrations of impurities and high possibilities of interferences influence of sample matrix. The method comprises the steps of providing a sample in which the components are contained, and depositing the components onto a working electrode, altering the environment of the working electrode so that it is immersed in a supporting electrolyte by effecting a matrix exchange and stripping the deposited electrochemically active components from the working electrode into the supporting electrolyte. While this technique decreases interference problems, it significantly complicates the design of the system and algorithm of measurements. The method and apparatus utilize a mercury drop electrode, and the stability and size of the hanging mercury drop electrode are critical for overall accuracy and precision of the analysis. Also, additional steps of removing the sample from the cell after deposition of electrochemically active species and pumping electrolyte to the cell may cause unwanted changes on the electrode surface, which decreases the accuracy and precision of the analysis, thereby increasing the time of the analysis.
The system and process described in U.S. Pat. No. 4,626,992, entitled, “WATER QUALITY EARLY WARNING SYSTEM” to Greaves, et al., is confined to the detection and identification, via video monitoring techniques, of living organisms in sources of water supplies. The computer includes two software programs, one is responsive to the measurements by the sensors to derive a set of prediction parameters corresponding to the statistical distribution of the expected movement patterns of the organisms. The other software program is used for analyzing the organisms movement and comparing the observed movements with the set of prediction parameters, and for initiating the generation of the warning message when the organisms observed movements do not correspond to the prediction parameters.
U.S. Pat. No. 4,723,511, entitled, “CONTINUOUS MONITORING OF WATER QUALITY” to Solman, et al., describes a slow monitoring system for rapid feed forward and feedback data mechanism to manage a modern water treatment system. The purity and presence of contaminants is monitored by the reactions of a fish in a tank of water.
U.S. Pat. No. 5,646,863 Morton, entitled, “METHOD AND APPARATUS FOR DETECTING AND CLASSIFYING CONTAMINANTS IN WATER” describes a system which samples, detects, measures, and reports, in near-real time, the presence of contaminants and thereby provides users with the ability to continually monitor conformance of water with established health and safety standards. This apparatus has ample measurement sensors selected from group consisting of pH sensor, temperature sensor, metal sensor, organic sensor, radiation sensor and biosensor. Stripping electrochemical sensors for measuring metals in parts per billion concentrations is claimed. The system and process of the exemplary embodiments measures ions, elements and compounds of metals, nonmetals and metalloids. The Morton system determines the voltammetric analysis oxidation current, which is related to the concentration in a sample.
U.S. Pat. No. 4,300,909, entitled “PROCESS CONTROL” to Krumhansl, relates to methods and apparatus for measuring the chemical state of a fluid and physical state of both the fluid and an apparatus for treating it. It provides that information to an algorithm solving apparatus, and accomplishing process action in response to signals from the algorithm solving apparatus. Krumhansl is related to a swimming pool water treatment application. The process control includes functions of measuring the state of contaminants in a fluid and the interaction between the data and the apparatus for treating it by furnishing that information to an algorithm solving apparatus to accomplish functional responses.
U.S. Pat. No. 5,292,423 of Wang, entitled “METHOD AND APPARATUS FOR TRACE METAL TESTING” is limited to microliter samples measurements for metal concentration using mercury-coated screen printed electrodes. The exemplary embodiments measure a wide range of elements, metals, metalloids, and nonmetals and their derivatives, using different electrochemical methods, such as using ion-selective electrode and voltammetrically using solid state graphite electrodes.
U.S. Pat. No. 5,873,990 to Wojciechowski, entitled “HANDHELD ELECTROMONITOR DEVICE” the portable monitor is a microprocessor based instrument designed to conventionally and rapidly measure various analytes in environmental and biological samples. The system uses battery or DC power. Unique electronic, microchip configurations were developed for the device to make it portable, low-cost, safe and simple to operate the instrument. The instrument has a small size, and the analysis is done on a manually taken sample. Calibration of the device using calibration strips is proposed. The colloidal gold electrode is applied for electrochemical measurements. The device is developed for metal analysis.